Advances in Optical Coherence Tomography (OCT) technology have made it possible to use OCT in a wide variety of applications. One application of OCT is in ophthalmology for imaging eye diseases due to the high transmittance of ocular media. OCT technology was invented in the early 1990's to generate depth-resolved images of tissue level microstructures, in vivo, and without physical contact. Second generation imaging technology, such as frequency-domain, swept-source, and spectral-domain OCT, has improved the signal-to-noise ratio over first generation technology, translating to faster imaging. As a result of this speed increase, high resolution cross-sectional images (B-scans) can be acquired at video-rates and three-dimensional images can be acquired very quickly. Sunita Sayeram and Joseph Izatt, “High-resolution SDOCT imaging—cutting-edge technology for clinical and research applications,” Photonik (November 2008) (hereinafter referred to as the “Photonik Article”).
As noted in the Photonik Article, OCT is an imaging technique which provides microscopic tomographic sectioning of biological samples. By measuring singly backscattered light as a function of depth, OCT fills a valuable niche in imaging of tissue ultrastructure, providing sub-surface imaging with high spatial resolution (−5-10 μm) in three dimensions and high sensitivity (>110 dB) in vivo with no contact needed between the probe and the tissue.
In biological and biomedical imaging applications, OCT allows for micrometer-scale imaging non-invasively in transparent, translucent, and highly-scattering biological tissues. As illustrated in FIG. 1, the longitudinal ranging capability of OCT is based on low-coherence interferometry, in which light from a broadband source is split between illuminating the sample of interest and a reference path in a fiber optic interferometer. The interference pattern of light backscattered from the sample and light from the reference delay contains information about the location and scattering amplitude of the scatterers in the sample. This information is recorded as a map of the reflectivity of the sample versus depth, called an A-scan.
For two or three-dimensional OCT imaging, multiple A-scans are acquired while the sample beam is scanned laterally across the tissue surface, building up a map of reflectivity versus depth and one or two lateral dimensions. The lateral resolution of the B-scan is given by the confocal resolving power of the sample arm optical system.
Ophthalmology has embraced minimally-invasive surgery since 1956 when the high-pressure xenon-arc lamp became commercially available for photocoagulation. This device has been replaced by various lasers developed over the years. As a result, laser procedures have tremendously advanced and improved vision outcomes in all segments of ophthalmic surgery.
The Mark-III FEL at Vanderbilt University operates in the 2-10 μm region with a 5 μsec macropulse containing a train of 1-ps micropulses at 3 GHz permitting wavelength selection for specific laser-tissue interactions. It has been determined that a wavelength of 6.1 μm or 6.4 μm produced by the FEL is capable of ablating tissue with a minimal amount of collateral damage, which is desirable for incisions of tissue. Tissues which have been examined with this wavelength include articular cartilage, fibro-cartilage, skin, cornea, and optic nerve sheath. The infrared energy can be delivered through small hollow-glass waveguides to permit the development of microsurgical and minimally invasive procedures. Other laser procedures are performed with conventional lasers with energy directed through laser fibers. An incising instrument would become more clinically valuable if the depth of the incision could be carefully monitored and controlled.